Calcite

Calcite is a carbonate mineral and the most stable form of calcium carbonate (CaCO₃). It is extremely common, especially as a main ingredient in limestone. On the Mohs scale, it defines hardness 3. Large calcite crystals are used in optical devices, and limestone, which is mostly calcite, has many practical applications. Two other forms of calcium carbonate are the minerals aragonite and vaterite. Aragonite converts to calcite within days or less at temperatures above 300 °C, and vaterite is even less stable.

The name comes from the German *Calcit*, a 19th-century term derived from the Latin *calx* (meaning lime, with genitive *calcis*), plus the mineral suffix *-ite*. It is thus a doublet of the word *chalk*. In archaeology and the stone trade, the term *alabaster* is sometimes used for a fine-grained, translucent, banded calcite deposit, though in geology and mineralogy *alabaster* is reserved for a variety of gypsum.

Two different sets of Miller indices are used to describe directions in calcite crystals: three indices (h, k, l) along the a₁, a₂, c axes, or four Bravais–Miller indices (h, k, i, l) along a₁, a₂, a₃, c, where i is redundant but helpful for visualizing symmetry. There are also two definitions of the unit cell. The older "morphological" unit cell was found by measuring crystal face angles with a goniometer; it is rhombohedral, with approximate dimensions a = 10 Å and c = 8.5 Å. The later "structural" unit cell, determined by X-ray crystallography, is hexagonal (a rhombic prism) with approximate dimensions a = 5 Å and c = 17 Å. To convert from morphological to structural units, c is multiplied by 4. For example, calcite cleavage is described as perfect on {1 0 1 1} in morphological coordinates and on {1 0 1 4} in structural coordinates; in {hkl} indices these are {1 0 1} and {1 0 4}, respectively. Twinning, cleavage, and crystal forms are often given in morphological units.

Diagnostic properties include a Mohs hardness of 3, a specific gravity of 2.71, and a vitreous luster in crystalline varieties. Color is white or colorless, but impurities can produce shades of gray, red, orange, yellow, green, blue, violet, brown, or even black.

Calcite has many crystal habits, combining over 1,000 possible crystallographic forms. The most common are scalenohedra (with faces in the hexagonal {2 1 1} or {2 1 4} directions) and rhombohedra (with faces in the {1 0 1} or {1 0 4} directions, which are also the main cleavage planes). Habits include acute to obtuse rhombohedra, tabular forms, prisms, and various scalenohedra. Several twinning types add to the variety. It can also occur as fibrous, granular, lamellar, or compact masses. A fibrous, efflorescent habit is called lublinite. Cleavage is in three directions parallel to the rhombohedron form. Fracture is conchoidal but hard to produce. Scalenohedral faces are chiral and come in mirror-image pairs; their growth can be influenced by chiral biomolecules like L- and D-amino acids. Rhombohedral faces are not chiral.

Calcite ranges from transparent to opaque and may show phosphorescence or fluorescence. A transparent variety known as Iceland spar is used in optics. Acute scalenohedral crystals are sometimes called dogtooth spar, and the rhombohedral form is sometimes called nailhead spar. The rhombohedral form may also have been the sunstone used by Viking navigators, as mentioned in the Icelandic Sagas. Single crystals display strong birefringence (double refraction), causing objects viewed through clear calcite to appear doubled. This effect was first described by Danish scientist Rasmus Bartholin in 1669. At about 590 nm, the ordinary and extraordinary refractive indices are 1.658 and 1.486, respectively. Between 190 and 1700 nm, the ordinary index ranges from about 1.9 to 1.5, and the extraordinary index from about 1.6 to 1.4.

Calcite is thermoluminescent, mainly due to divalent manganese (Mn²⁺) impurities. In an experiment, activators such as ions of Mn, Fe, Co, Ni, Cu, Zn, Ag, Pb, and Bi were added to calcite samples to see if they emitted heat or light. Ions of Cu⁺, Cu²⁺, Zn²⁺, Ag⁺, Bi³⁺, Fe²⁺, Fe³⁺, Co²⁺, and Ni²⁺ did not cause a reaction. However, a reaction occurred when both manganese and lead ions were present. By changing the temperature, the glow could be observed.